1. Field of the Invention
The present invention relates to a rolling bearing unit with a rotation speed detection instrument for use in cars, and to a method for working an outer race for use in the bearing unit.
Particularly, the present invention relates to improvements in a rolling bearing unit with a rotation speed detection instrument which allows a wheel of a car to be supported ratatably on a suspension device thereof and which is used to detect a rotation speed of the wheel, as well as to an outer race constituting the rolling bearing unit with the rotation speed detection instrument.
2. Related Background Art
For rotatably supporting a wheel of a car onto a suspension device, a rolling bearing unit is used. The rotation speed of the wheel must be detected to control an antilock braking system (ABS) as well as a traction control system (TCS). For this reason, it has been widely performed that the foregoing wheel is rotatably supported onto the suspension device by a rolling bearing unit with a rotation speed detection instrument which is built therein and the rotation speed is detected.
Such rolling bearing unit with a rotation speed detection instrument supports a hub on the inner diameter side of its outer race rotatably interposing a plurality of rolling elements between the outer race and the hub.
At the same time, the rotation speed of an encoder fixed to a portion of the hub is detected by a sensor which is supported by a portion of the outer race. The characteristic of the encoder in the circumference direction is changed alternately at equal intervals. Moreover, the technology that the detection portion of the sensor is made to face the outer peripheral surface of a large diameter cylindrical portion of the encoder at a situation where the sensor is supported in a mounting hole formed at an intermediate portion of the outer race in its axial direction and to be elongated in the diameter direction of the outer race, has been widely known as disclosed in, for example, Japanese Patent Laid-open No. 63-59769, 6-109027, 8-210659, and many publications.
When the roiling bearing unit with the rotation speed detection instrument as described above is used, the foregoing outer race is supported on the suspension device, the wheel is fixed to the end portion of the hub, which portion is located a part from the outer race, thereby the wheel being supported rotatably on the foregoing suspension device. When the foregoing encoder rotates with the rotation of the wheel, the output of the sensor having the detection portion facing the encoder changes. The frequency in which the output of the sensor changes is proportional to the rotation speed of the wheel. Therefore, if the output signal of the sensor is sent to a controller which obtains the rotation speed of the wheel, the ABS as well as the TCS can be controlled suitably.
In the case of the rolling bearing unit with the rotation speed detection device described in the foregoing gazettes, no consideration for the compatibility of the workability of the mounting hole with a security of the durability of the outer race was made. Specifically, in order to secure the rolling fatigue life time of the track or raceway of the outer race, a hardened layer having the hardness more than 500 Hv must be formed in the inner peripheral surface of the enter race, and in the surface of the outer race where the outer race tracks are formed and in the vicinity of that surface. This layer is formed to a depth (about 1.5 mm in the case of the rolling bearing unit of cars) more than three times inclusive of three times as large as a depth where a shearing force based on a stress applied by the rolling element becomes maximum (about 0.5 mm in the case of the rolling bearing unit of cars). A transition layer of about 0.5 mm thick exists between the deepest portion of this hardened layer and its so-called raw portion which is not hardened. In this transition layer portion, the hardness gradually decreases from the hardened layer to the raw portion.
In order to form the hardened layer in each of the outer race track portion, although the control of the hardening depth with respect to the thickness direction was considered, the regulation of the formation range of the hardened layer in the surface direction of the inner periphery of the outer race, particularly, in the axial direction, was not considered. Specifically, with respect to the thickness direction, in order to secure the toughness of the outer race, although to leave the raw portion in the vicinity of the outer periphery of the outer race was considered (see, for example, Utility Model Gazette No. 2529597), the consideration in the surface direction was not considered. As described above, when the thickness direction of the hardened layer is considered concerning the range of the hardened layer and the hardened layer is formed in each of the foregoing outer race track portions without considering the surface direction of the hardened layer, the range of the hardened layer may reach to about 10 mm from the end portion of each outer race track or more. The hardness of the hardened layer becomes high in the portion of about 10.5 mm or more apart from the end of each outer race track.
On the other hand, in response to the demand for saving fuel consumption of the cars in recent years, to downsize the rolling bearing unit for supporting the wheel and to reduce a weight of it, the shortening of the dimension of this rolling bearing unit in the axial direction has been developed. Based on such shortening, the distance between the end periphery of each cuter race track portion and the mounting hole becomes short, and this distance sometimes can not be kept to be more than 10.5 mm. For this reason, the hardness in the portion where the foregoing mounting hole is to be formed becomes high too, so that working of this mounting hole will be cumbersome. Although the working of this mounting hole may be well performed before the formation of the hardened layer in each outer race track portion, when the hardened treatment is performed after the working of the mounting hole, the considerations for preventing the occurrence of defects due to stress concentration in the mounting hole portion is needed, resulting in troublesomeness of the hardening treatment.
Furthermore, in Utility Model Gazette No. 2529597, the technology is disclosed, in which the hardened layer is formed also in a portion between a plurality of the outer race tracks of the outer race in a part of the inner peripheral surface of the outer race, and the portion between the outer race tracks is made to be not liable to occur a plastic deformation. As described above, when the portion between the outer race tracks is made to be not liable to occur the plastic deformation, the plastic deformation of the outer race is prevented, so that an increase in durability of the rolling bearing unit with the rotation speed detection instrument including this outer race can be achieved, in spite of the fact that an impact load is applied to the outer race due to crashing of the wheel to paving stones. It should be noted that even in such case, toughness of the outer race is secured while leaving a raw portion that is not hardened in the vicinity of the outer periphery of the outer race and occurrence of damages of the outer race such as cracks due to the application of the foregoing impact load can be prevented.
However, when the hardened layer is formed also between the outer race tracks in the part of the inner peripheral surface of the outer race, working of the mounting hole to fixedly support the sensor, which is formed in the outer race, becomes difficult.
On the other hand, in response to the demand for saving fuel consumption of the cars in recent years, to downsize the rolling bearing unit for supporting the wheel and to reduce a weight of it, the shortening of the dimension of this rolling bearing unit in the axial direction has been developed. Based on such shortening, the distance between the end periphery of each outer race track portion and the mounting hole becomes short, and this distance sometimes can not be kept to be more than 10.5 mm. For this reason, the hardness in the portion where the foregoing mounting hole is to be formed becomes high too, so that working of this mounting hole will be cumbersome. Although working of this mounting hole may be well performed before the formation of the hardened layer in each outer race track portion, when the hardening treatment is performed after the working of the mounting hole, the considerations for preventing the occurrence of defects due to stress concentration in the mounting hole portion is needed, resulting in troublesomeness of the hardening treatment.
Furthermore, as described in the foregoing Utility Model Gazette No. 2529597, the technology has been known, in which the hardened layer is formed also between a plurality of the tracks of the outer race in order to enhance the durability of the rolling bearing unit with the rotation speed detection instrument including the outer race by making the outer race not liable to occur the plastic deformation, even when an impact load is applied to the outer race through the hub and the rolling element owing to reasons such as crashing of the wheel against the paving stone. As described above, in the case where the hardened layer is formed also between a plurality of the outer race tracks of the outer race, the working of the foregoing mounting hole becomes cumbersome, not only in the case of small-sized rolling bearing units but also even in the case of comparatively large-sized rolling bearing unit with the rotation speed detection instrument in which the distance between the end periphery of each outer race track portion formed in the inner periphery surface of the outer race and the mounting hole, is relatively large.